Steve, how does one know the amp output impedance without a bench of electronic tools? Is there a way with just a VOM?
Hi Scott,
There is a way which involves measuring a sine wave (usually 400 or 1khz, but any lower F is ok for our purposes, say 60hz) with out a load. In triode mode the SE amp only puts out ~1.8 watts. For that amp, a 10 watt 2 to 5 ohm rheostat, or 2 watt 2 to 5 ohm resistor should work.
I would not worry about the voltage meter loading the amplifier's output.
Now adjust the rheostat until the output voltage measures 1/2 the no load voltage. At such point the output impedance (Z) equals the load/pot Z.
One probably won't have a rheostat, so one could measure the No load output voltage, and then place a low ohmage fixed resistor, as mentioned above, across the output.
Now measure the voltage with the load connected. Just as before, a voltage divider, a voltage ratio is created, so a resistance ratio is created.
In general, for any newbies out there, this example.
Say we measure 3 volts no load. Now we add a fixed 1.5 ohm load resistor, and measure 2 volts output.
We now know that 2 volts is across the 1.5 ohm load, so 1 volt is across the output impedance of the amplifier, to equal 3 total volts.
1 volt across the amplifier output Z and 2 volts across the load. That is a 1 to 2 ratio. We know the value of the load resistance, 1.5 ohms. So 1/2 of that is 0,75 ohms. That is the approximate value of the amplifier output Z.
The last question is, how do we know the amplifier is not clipping, which could influence the reading? I would listen to a tone (say from YouTube) that is loud but not clipping. That should be close enough.
I hope this helps Scott.
steve